1. Field of the Invention
The present invention relates to an active matrix flat panel display, more particularly, to an organic light emitting diode capable of generating proper luminance and extending life cycle by controlling the amount of current flowing through an organic electroluminescent (EL) device per unit pixels.
2. Description of Related Art
Although an active matrix organic light emitting diode (AMOLED) that is a flat panel display gradually requires a high-resolution panel, there have been much difficulties fabricating the high-resolution panel due to characteristics of the organic electroluminescent device and the driving transistor for driving the organic electroluminescent device.
For example, in case of a 5″ WVGA AMOLED having pixel size of 45.5 μm×136.5 μm and resolution of 180 ppi or more, luminance of 50 cd/m2 per unit area is generated, and it is preferable that a proper amount of current flows to the EL (electroluminescent) device per unit pixel to generate the luminance since luminance per unit area is greatly increased by current amount of the limit value or more if the amount of current flowing to the EL device per unit pixel exceeds a limit value, and the life cycle of the EL device is rapidly decreased accordingly. Therefore, it is preferable that a proper amount of current for generating a certain luminance per unit area flows through the EL device to radiate a pixel, namely, an EL device.
FIG. 1 illustrates an equivalent circuit diagram for a unit pixel in a conventional active matrix organic light emitting diode. Referring to FIG. 1, a conventional organic light emitting diode 100 includes unit pixel 150 connected to gate line 110, data line 120 and common power line 130. The unit pixel 150 includes two p type thin film transistors (TFTs) of switching transistor 151 and driving transistor 155, one capacitor 153 and one organic electroluminescent (EL) device 157.
The switching transistor 151 is driven by scan signals provided to the gate line 110 to switch data signals provided to the data line 120. The driving transistor 155 determines the amount of current flowing through the EL device 157 according to the data signals transmitted through the switching transistor 151, namely, a voltage difference (Vgs) between gate and source. The capacitor 153 plays a role of maintaining a voltage difference (Vgs) between the gate and the source of the driving transistor 155.
FIG. 2 illustrates a plane structure of driving transistor in a conventional organic light emitting diode.
Referring to FIG. 2, a conventional driving transistor comprises a semiconductor layer 220, gate electrode 240 and source/drain electrodes 261 and 265. The semiconductor layer 220 includes a channel region 224 formed on a part corresponding to the gate electrode 240, and high concentration source/drain regions 221 and 225 formed on both sides of the channel region 224. The source/drain electrodes 261 and 265 are electrically connected to the high concentration source/drain regions 221 and 225 through contact holes 251 and 255.
A TFT fabricated using a low temperature polysilicon film is used as the driving transistor, wherein the low temperature polysilicon film is suitable for a current driving type AMOLED (active matrix organic light emitting diode) since the low temperature polysilicon film has a high mobility and a low off current as illustrated in FIG. 9. The amount of current flowing to the EL device through the driving transistor is increased, and luminance is excessively increased since size of pixel is reduced to result in a reduction of size of the anode electrode in an AMOLED of 180 ppi or more. Finally, there are problems in the AMOLED of 180 ppi or more that life cycle of the EL device is reduced as current density per unit area is increased. Namely, referring to FIG. 9 illustrating a relation of drain current (Id) to gate voltage (Vg) of the driving transistor, the amount of current of 1 μA or more that is about 1-order larger than 90 nA, an on current value suitable for light emitting diodes of 180 ppi or more, flows through the driving transistor 155 in a conventional organic light emitting diode. Therefore, there are problems in that on current of the driving transistor should be reduced to obtain luminance suitable for an AMOLED of 180 ppi or more.
On the other hand, there are problems of high leakage current although the amount of current flowing to the EL device through the driving transistor can be reduced if a thin film transistor (a-Si TFT) formed of amorphous silicon is applied to driving transistor of the AMOLED.